The problem is, where do you draw the line between legitimate criticisms and conspiratorial nonsense?

Again, the Zionist example: there are definitely criticisms of Zionism which are nothing more than baseless antisemitic conspiracy theories (e.g. "Zionist Occupation Government"). But, that doesn't mean all criticisms of it are so baseless. If you read pro-Palestinian authors, or even left-wing Jewish Israelis, you will encounter critiques of Zionism, which whether they ultimately be right or wrong, can't be dismissed on the same grounds.

You will also find claims that some people on the pro-Israel side want to erase the difference between non-antisemitic criticisms of Zionism and antisemitic criticisms of Zionism, in order to put Zionism itself beyond criticism – and I think sometimes those claims are correct. On the other hand, sometimes pro-Palestinian criticisms of Zionism do cross the line into antisemitism, and as applied to those particular cases the claim is false. Part of the problem here, is it isn't always clear where to draw the line between non-antisemitic and antisemitic criticisms of Zionism–it is a question about reasonable people can disagree. Non-antisemitic criticism of Zionism and antisemitic criticism of Zionism aren't totally disjoint, they bleed into each other at the edges.

And I think we have a similar situation with "cultural Marxism". Yes, Breivik's criticisms of it are conspiratorial nonsense – but is everyone else's? There are right-wing criticisms of it which, whether correct or incorrect, arguably aren't "conspiratorial nonsense" – see for an example, see the 2018 article by the conservative lawyer Alexander Zubatov in the Jewish magazine Tablet, "Just Because Anti-Semites Talk About ‘Cultural Marxism’ Doesn’t Mean It Isn’t Real" [0]. There are even left-wing criticisms of it – the cultural Marxist desire to switch the focus of Marxism from economic to cultural issues was (and still is) heavily criticised by orthodox Marxists who view that switch as a distraction and a mistake.

And I see two other parallels to the Zionism case: firstly, just as some Zionists arguably seek to dishonestly erase the distinction between non-antisemitic and antisemitic criticisms of Zionism, in order to put it beyond criticism – in a similar way, my own impression is that some progressives seek to erase the distinction between conspiratorial (even antisemitic) criticisms of cultural Marxism, and non-conspiratorial (even if possibly mistaken) conservative criticisms of it, in order to shut down debate.

And secondly, just as the boundary between non-antisemitic and antisemitic criticisms of Zionism is open to debate, I think the same is true for the boundary between non-conspiratorial and conspiratorial criticisms of cultural Marxism. In my own mind, Zubatov is clearly on the "could well be wrong but not conspiratorial" side of that line, and Breivik is definitely on the "conspiratorial" side of the line – but I'm less clear about where to put something like the Heritage Foundation's criticisms of it. [1] And that's the core problem with your idea that we consider conspiratorial criticisms and non-conspiratorial criticisms of "cultural Marxism" to be talking about two completely unrelated things – it assumes a clearcut boundary between the two which may not actually exist. I think it makes more sense to speak of a continuum of criticisms of the one thing (ranging from the reasonable to the ludicrous, with the boundary between the two being debatable) rather than claiming the reasonable criticisms and the ludicrous criticisms are criticising two completely different things.

[0] https://www.tabletmag.com/sections/news/articles/just-becaus...

[1] https://www.heritage.org/sites/default/files/2022-11/SR262.p...